Improving the spatial resolution of epiretinal implants by increasing stimulus pulse duration

Retinal implants that deliver longer pulses of electrical current may markedly improve image sharpness for blind individuals. Sharper sight with sustained stimuli As their photoreceptors degenerate, people with retinitis pigmentosa lose their vision. The retinal implants now in clinical use can help; these devices respond to light and deliver an electrical stimulus directly to the remaining cells in the retina and thereby to the visual centers of the brain. But current technology only enables patients to sense the presence or absence of light or, for some, large objects. Weitz et al. show that increasing the duration of the stimulus to the residual cells can markedly improve performance. Experiments in isolated retinas from an animal model reveal why this works: longer stimuli do not inadvertently activate passing axons (which can muddy the signal). In one patient with an epiretinal implant, longer-duration stimuli allowed visualization of distinct focal spots of light. Retinal prosthetic implants are the only approved treatment for retinitis pigmentosa, a disease of the eye that causes blindness through gradual degeneration of photoreceptors. An array of microelectrodes triggered by input from a camera stimulates surviving retinal neurons, with each electrode acting as a pixel. Unintended stimulation of retinal ganglion cell axons causes patients to see large oblong shapes of light, rather than focal spots, making it difficult to perceive forms. To address this problem, we performed calcium imaging in isolated retinas and mapped the patterns of cells activated by different electrical stimulation protocols. We found that pulse durations two orders of magnitude longer than those typically used in existing implants stimulated inner retinal neurons while avoiding activation of ganglion cell axons, thus confining retinal responses to the site of the electrode. Multielectrode stimulation with 25-ms pulses can pattern letters on the retina corresponding to a Snellen acuity of 20/312. We validated our findings in a patient with an implanted epiretinal prosthesis by demonstrating that 25-ms pulses evoke focal spots of light.